Transmission line type microwave filter



Oct. 3, 1967 G. c. DI PIAZZA 3,345,589

TRANSMISSION LINE TYPE MICROWAVE FILTER Filed Dec. 14, 1962 2Sheets-Sheet 1 PIP/OR APT F/GIZ a /NVE/VTOR 6. C. D/ P/AZZA ATTORNEY,

0ct. 3, 1967 G. c. DI PIAZZA TRANSMISSION LINE TYPE MICROWAVE FILTER 2Sheets-Sheet 2 Filed Dec. 14, 1962 United States Patent Office 3,345,589Patented Oct. 3, 1967 3,345,589 TRANSMISSION LINE TYPE MICROWAVE FILTERGerald C. Di Piazza, Lake Hiawatha, N.J., assignor to The presentinvention relates to frequency selective net- Works, and morespecifically to narrow-band microwave filters.

In many microwave applications, it is desirable to transmit a narrowband of frequencies while simultaneous- 1y suppressing all otherfrequencies. Prior art filters for accomplishing this end have takenmany and varied forms including the well known capacity-coupledhalf-wave resonator structures. Examples of such devices are disclosedin United States Patents Nos. 2,859,417 and 2,867,782, both granted toM. Arditi on Nov. 4, 1958, and J an. 6, 1959, respectively.

Most of the prior art filters, due largely to the inherent limitationsof the resonator structures employed, suffer one or more practicaldisadvantages. These disadvantages often include high inband insertionloss, relatively low selective capability and relative difficulty ofdesign or construction. It is therefore an object of the presentinvention to reduce the inband insertion losses in narrow-band microwavebandpass filters.

It is a further object of the present invention to increase theselective capabilities of microwave bandpass filters.

It is another object of the present invention to simplify the design andconstruction of high selectivity microwave bandpass filters.

The foregoing objects are accomplished in accordance with the principlesof the present invention by utilizing a transmission line having auniform characteristic admittance across which there are shunt-connectedtransmission line elements having characteristic admittances greaterthan that of the uniform line. As used herein, the term transmissionline is understood to mean any structure capable of supportingpropagating high frequency electromagnetic wave energy. Such structurescan include coaxial lines, strip transmission lines, parallel wiretransmission lines and conductively bounded waveguiding structures. Theshunt transmission line elements are open-circuited at one end andshort-circuited at the other end. The connection between the uniformline and each shunt line is at a point intermediate these two ends. Ashunt transmission line element connected in this fashion is known as aftapped stub. The ratio of the length of the open stub section to thetotal stub length is known as the tapping ratio.

By utilizing the so-called loaded Q approach, it has been found that, asthe ratio of stub-to-line characteristic admittance is increased, theselectivity of the filter is likewise increased. The selectivity of thefilter of the present invention can be further increased by increasingthe tapping ratio mentioned above.

In another embodiment of the invention, even greater selectivity isobtained by the utilization of a branched transmission lineconfiguration. In this embodiment, a first uniform transmission linebranches into a plurality .of secondary uniform transmission lines. Ingeneral, the characteristic admittance of each branching line issubstantially equal to that of' the first uniform line divided by thenumber of branching lines. Frequency selective stu'bs or circuits areshunt-connected across each of the secondary lines. The secondarytransmission lines again converge to form a uniform transmission linehaving a characteristic admittance substantially equal to that of thefirst uniform line.

A distributed shorting element for conductively shorting a section of astrip transmission line or stub to a pair of extended conductive groundplanes is also described. This element consists of a thin piece ofconductive material bent into the form of an E and held in physical andelectrical contact with the ground planes and center conductive strip byexternal clamping means.

The above mentioned and other features and objects of the presentinvention will become more apparent by reference to the followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic illustration of a transmission line filter of thetype well known in the prior art;

FIG. 2 is a schematic illustration of a two-element bandpass filter inkeeping with the principles of the present invention;

FIG. 3 is a graphical representation of the tapping ratio of the shuntstubs of the embodiment of FIG. 2 plotted as a function of normalizedfrequency for various ratios of stub-to-line characteristic'adunittances;

FIG. 4 is a pictorial view of the embodiment of FIG. 2 adapted for astrip transmission line configuration;

FIG. 5 is a pictorial illustration of a second embodiment of the presentinvention adapted for strip transmission line configuration;

FIG. 6 is a pictorial View of an improved distributedtype shortingelement capable of being employed in stirp transmission line devices;

FIG. 7 is an exploded pictorial view of a portion of the embodiment ofFIG. 4 showing the manner in which the distributed shorting element ofFIG. 6 is utilized; and

FIG. 8 is a cross-sectional view of the embodiment of FIG. 4 showing thedistributed shorting element of FIG. 6in place. 1

Referring more specifically to the drawings, FIG. 1 is a schematicillustration of a transmission line filter of the type well known in theart. In this structure, a plurality of transmission line stubsdesignated 1, 2, 3 n are connected at one-quarter wavelength intervalsto a uniform transmission line 10. The total Q of the filter Qrepresents a combination of the Qs of the constituent sections. These Qsare designated Q Q Q Q and comprise theQof each stub plus thecompensation for the selectivity introduced by the sections of line 10immediately adjacent to each stub. It is well known that the Qattributable to a one-quarter wavelength section of uniform line in sucha filter can be approximated, in a narrow-band filter, by the additionof the factor 1r/8 to the Q of each stub. (See US. Patent No. 2,540,488granted to W. W. Mumford on Feb. 6, 1951.)

3 where Q Q Q Q represents the Q of each stub 1, 2, 3, n, respectively.

In the above cited patent of W. W. Mumford, it is stated that for amaximally-flat bandpass filter, the Q of any section is given by Q.=QTsin 2) where r denotes the order of the stub. (See also US. Patent No.1,849,656 granted to W. R. Bennett on Mar. 15', 1932.)

In terms of the characteristic admittance, it can be shown that where Yis the characteristic admittance of line and Y is the characteristicadmittance of the rth stub.

Since it is a useful parameter in the design of filters in accordancewith the present invention, the ratio Y /Y will be designatedhereinafter as k From Equation 3 it is obvious that the Q of a stubincreases as its characteristic admittance Y increases. In general,therefore, high Q is characterized by high stub admittance.

It has been found that the use of a tapped stub configuration intransmission line filters yields higher Qs than the shorted stubconfiguration of FIG. 1. A schematic diagram of a two element tappedstub bandpass filter is shown in FIG. 2. In general, any number of stubscan be utilized in practicing the present invention; however, for thepurposes of illustration, a filter utilizing two stubs is described.

The bandpass filter of FIG. 2 consists of a length of uniformtransmission line having a characteristic admittance Y across which areconnected tapped transmission line stubs 21 and 22. Stubs 21 and 22,which have characterstic admittances Y and Y respectively, are connectedin shunt with line 20; and spaced apart onequarter wavelength at themidfrequency of the band of frequencies to be passed. The overall lengthof each stub 21 and 22 is also one-quarter wavelength at said midbandfrequency, although as will be discussed in greater detail hereinbelow,the length and spacing of these stubs can be any odd multiple ofone-quarter wavelength. The connections between line 20 and stubs 21 and22 are made at points intermediate their respective ends. Each stub isopen circuited at one end and short circuited at the other end. Theelectrical distance from the tapped connection to the shorted end ofeach stub is designated 1 and the distance between this connection andthe open end is designated From Equation 2 the Qs of stubs 21 and 22 areand since Y is constant, it is seen from Equation 3 that o2= o1- Thevalue of susceptance due to stub 21 or 22 at the junction of line 20 andthe stub is described by where [3 is the phase constantof the stub andis equal to 21r/)\, where A is the wavelength measured along the stub.The normalized susceptance B can be found by dividing Equation 6 by thecharacteristic admittance Y of line 20.

- J'n 1;L ]BN Y0 tan [3Z2 Y0 001,51 I and since 1/01- &2 'YOIC1-YOICZ kthen tan cot fil Since the tapping ratio a l /l, then l =(1a)l, and

B =k[taIl aBZ-COt (1oz)Bl] 1+tan Bl tan aBZ l: am tan BZtan 0431 B tanaBZ+l tan aBl-tan s2 9) Solving for tan 0:51,

It can further be shown that at the half power points the normalizedsusceptance B of the stub is equal to two. Substituting this value of Bin Equation 10 gives where 5,, is the phase constant of the stub at thecutoff or half power points. Since l= /4,

where f and f are the frequencies corresponding to midband and cutoffrespectively.

Substituting and solving for or 1/2 l[1k 2k tan tan 1 As shownhereinabove, the loaded Q or each stub section can be increased byincreasing the ratio k while keeping the other variables constant. Theselectivity of a bandpass filter in accordance with the presentinvention, therefore, can be improved by making the ratio k larger thanunity. However, since the relationship between k and the loaded Q is asmooth function this improvement is gradual. As will be discussed ingreater detail hereinbelow, it is generally desirable to make k as largeas possible within limits imposed by the physical dimensions of thefilter structure; although in a practical filter, a significantimprovement is obtained when the ratio k is larger than two.

FIG. 3 is a graphical representation of the tapping I ratio a plotted asa function of f /f for values of k equal to 1.0, 2,5, 3.33, and 5.0. Thecurves were obtained by inserting the appropriates values of k and f /finto Equations 13 and 14 and solving for a. From a physical standpoint,the graph of FIG. 3 indicates that, for a given bandwith, a filter canbe realized by choosing either a relatively high tapping ratio, a, andlow k, or a relatively low a and high k.

As an illustration of the design procedure, the following example isgiven. It is the objective of the design to obtain a two-sectionmaximally-flat narrow-band band- :pass filter having a center frequencyof 2220 megacycles per second and an attenuation of at least 10 decibelsat afrequency of 2100 megacycles per second. The filter is a) to have aninsertion loss as small as possible at the center frequency.

The loss function of a maximally-flat filter is given where again n isthe number of stubs. By inserting the design values into Equation 15,the total Q, Q of the filter can be obtained, Thus,

n 1 minim g P i QT 2220 2100 (16) In other words, a two-section filterhaving a total Q of 16 will satisfy the specified design requirements.In order to provide a certain design margin, a convenient bandwith of100 megacycles per second shall be chosen for the filter. At a centerfrequency of 2220 Inegacycles per second, this bandwith corresponds to aQ of 22.2.

The schematic diagram of the illustrative filter corre sponds to thatshown in FIG. 2 and the nomenclature and numbers of the various elementsare therefore carried over to this example.

Substituting into Equation 2, the Q of each section is,

(2 :22.2 sin 7r/4=l57 (17) Q =22.2 sin 31r/4=15.7 (18) These values arethen substituted into Equation 1 to determine the Qs of the stubs.

= 0.065 normalized bandwidth In this example, it is assumed that k and kequal 2.5. Therefore, from FIG. 3 the tapping ratio a is approxi mately0.83. If a more exact ratio of a tapping ratio is desired, it can beobtained from Equations 13 and 14.

At this point, it should be noted that the curves of FIG. 3 are notquite symmetrical about the center frequency f For this reason, aparticular value of a will not yield a value of f which correspondsexactly with the arithmetic mean of the upper and lower cutofffrequency. It is seen, however, that for stubs of high Q this differencebetween the desired and the actual values of t is negligible.

Once the values of k and a have been determined, it is a relativelysimple matter to realize the desired filter. The type of transmissionline structure (for example, coaxial line, waveguide, strip transmissionline, etc.) should be chosen to meet the application for which thefilter is intended. The characteristic admittance of uniform line isgenerally dictated by the characteristic admittance of the connectinglines of the utilization circuit.

Because of the many advantages enjoyed by strip trans mission linestructures, further illustrative embodiments of the present inventionwill be shown with reference to such structure.

A strip transmission line filter, constructed in accordance with theteachings of the present invention, is illustrated in the pictorial viewof FIG. 4. In FIG. 4 a uni form strip transmission line 40 having tappedstubs 41 and 42 is bonded, deposited or etched on a dielectric sheet 43.The dielectric sheet 43 can comprise any suitable low loss material. Forexample, the materials known commercially as Teflon or Tellite have beensuccessfully employed in these structures. A conductive ground plane 44is bonded to the bottom side of sheet 43. A second sheet of dielectricmaterial 45, having conductive ground plane bonded to its upper surface,is positioned above sheet 43. For the purpose of clarity in FIG. 4, thetwo dielectric sheets are shown separated. In practice, however, it isunderstood that the two sheets would be clamped together by cover platesor other securing means well known in the art.

Shorting eyelets or rivets 47 are utilized to short circuit the bottomportion of stubs 41 and 42 to ground plane 44. A similar set of eyelets48% extend from the upper ground plane 46 through dielectric sheet 45 tocontact eyelets 47 and thereby short stubs. 41 and 42 to the upperground plane.

The entire filter structure of FIG. 4 therefore resembles a sandwichwherein the center conductor and the stubs are sandwiched between twoconductive ground planes separated by insulating sheets. In such astructure, the characteristic admittances of the center conductor andstubs are functions of their widths and thicknesses as well as thedielectric constant of the insulating sheets and the spacing between theground planes. The relation between these variables is given inReference Data for Radio Engineers, 4th edition, International Telephone& Telegraph Corporation, New York, 1956, pages 598600.

In the embodiment of FIG. 4, the thickness of the dielectric sheets 43and 45 is inch. The dielectric constant of the material of sheets 43 and45 is 2.25 over the frequencies of operation. Although, for the sake ofclarity, the thicknesses of the ground planes and conductive strips areshown in exaggerated proportion in the drawings, their actualthicknesses are negligible compared to the thicknesses of the dielectricinsulating sheets. These dimensions are primarily determined by thecommercial availability of the conductively clad dielectric insulatingsheets. With the dielectric constant specified, the length of stubs 41and 42 was determined as 0.88 inch. This corresponds to one-quarterwavelength at the center frequency of 2220 megacycles per second. Sincethen . l =0.732 inch (22) and l =0.148 inch (23) The spacing d betweenstubs 41 and 42 likewise corresponds to one-quarter wavelength atfrequency Due to the fringing elfects associated with the finite widthof the stubs, however, it was determined experimentally that the spacingshould be increased a few percent for optimum results. The spacingfinally employed was 0.905 inch. The characteristic admittance of theuniform line 40 was chosen as 1 mho which corresponds to a line width of0.10 inch. Since the characteristic admittance of stubs 41 and 42 equalsk times that of the line, or mho, their width are 0.33 inch.

As shown above, high Q is associated with a high ratio k of stub-to-linecharacteristic admittance. With a filter having the physical structureof the embodiment of FIG. 4, however, the Q can only be increased to acertain point. This is so because as Q increases, k must increase, andwith a transmission line of a given characteristic admittance, a greaterk corresponds to a greater stub width. When the width of the stubsbecomes significant compared to the one-quarter wavelength spacingbetween them, undesirable fringing effects and interaction between thetwo stubs occur.

One solution of this problem would be to increase the spacing betweenthe stubs to a higher odd multiple of one-quarter wavelength (forexample, three-quarter wavelength). This, however, has the obviousdisadvantage of making the overall length of the filter much longer. In

addition, such a structure has higher losses and a spuri- 'ous resonancepoint corresponding to that frequency at which the spacing equalsone-quarter wavelength.

Another solution is to construct the filter in accordance with'theembodiment illustrated in the pictorial view of FIG. 5. The principalfeatures of the filter of FIG. are its lower insertion loss and itsgreater selective capabilities. In the embodiment of FIG. 5 there isshown a conductive ground plane 50 bonded to the lower surface of adielectric insulating sheet 51. A uniform transmission line 52 having agiven characteristic admittance is bonded, deposited or etched on theupper surface of insulating sheet 51. At junction 53 along its length,line 52 branches into two separate but substantially identical lines 54and 55 which again merge into line 52 at junction 58. The dimensions oflines 54 and 55 are proportioned so that their characteristicadmittances are substantially equal to one-half the characteristicadmittance of line 52.

A pair of quarter-wave tapped stubs 56 are shunted across line 54 andspaced apart one-quarter wavelength at the center frequency. Asubstantially identical pair of stubs 57 are similarly connected acrossline 55. The characteristic admittances and tapping ratios on of stubs56 and 57 are determined in accordance with the design procedure givenabove except that the uniform line used as a reference is line 52 ratherthan branching lines 54 and 55.

Although in the embodiment of FIG. 5, the upper ground plane andinsulating sheet are not shown, it is understood that in order toprevent radiation and accompanying losses, it is generally advantageousto utilize these elements in a manner similar to that shown inconnection with the filter structure of FIG. 4.

Although only one embodiment is illustrated in FIG. 5, it is understoodthat it is susceptible to many modifications. For example, more than twobranching transmission lines may connect the two sections of uniformline 52. In such cases, the characteristic admittance of each of thebranching lines is substantially equal to that of the uniform linedivided by the number of branching lines. Likewise the frequencyselective portions of each branching line may take a form other thanthat of the tapped stubs shown in FIG. 5. Thus, tapped stubs 56 and 57can be replaced by untapped stubs or other frequency selective microwavecircuits known in the art.

Furthermore, if the electrical length of each of the branching lines ismade equal to an integral multiple of one-half wavelength at the midbandof the band of frequencies to be passed, their characteristicadmittances canassume a value other than the described fraction of thecharacteristic admittance of the uniform line.

In the embodiment of FIGS. 4 and 5, the shorted ends of the tapped stubsare shown connected to the respective ground planes by means of smallrivets or eyelets. FIG. 6 shows in pictorial view an alternative devicefor use in conductively shorting a portion of a strip transmission lineto the upper and lower ground planes. Element 60 consists of a thinrectangular piece of conductive material, such as copper, bent into theform of an E. The width of this element is preferably equal to the widthof the stub to be shorted.

FIG. 7 is an exploded pictorial view of a portion of the striptransmission line filter of FIG. 4 wherein shorting element 60 hasreplaced eyelets 47 and 48. Like numbers have been carried over fromFIG. 4 to FIG. 7 to correspond with like structural elements. Blocks 70and 71 have been cut from the upper and lower dielectric sheetsrespectively to facilitate insertion of element 60.

FIG. 8 is a cross-sectional view of the resulting structure with element60 and blocks 70 and 71 in place. As

element 69 and the ground planes 44 and 46 have been greatlyexaggerated. In practice, these thicknesses are on the order of a fewthousands of an inch. As a consequence, there is little if any gapbetween the strip transmission line and the lower surface of dielectricsheet 45.

Although certain specific embodiments of the invention have been shownin the drawings and described in the foregoing specification, it isunderstood that the invention is not limited to those specificembodiments, but is capable of modification and rearrangement by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed is:

1. A filter capable of passing microwave energy over a given band offrequencies comprising, in combination, a uniform transmission linehaving a given characteristic admittance, a plurality of tappedtransmission line stubs shunted across said line at intervalssubstantially equal to an odd multiple of one-quarter wavelength at themidfrequency of said given band of frequencies, each of said stubshaving a length substantially equal to an odd multiple of one-quarterwavelength at said midfrequency, each of said stubs having oneopen-circuited and one short-circuited end, the connection between saiduniform line and each of said stubs being intermediate said ends, andwherein the characteristic admittance of each of said stubs issubstantially greater than that of said uniform line.

2. A microwave filter comprising, in combination, a first and seconduniform transmission line section having a given characteristicadmittance, a plurality of branching transmission lines connectedbetween one end of said first transmission line section and one end ofsaid second transmission line section, the characteristic admittance ofeach of said branching lines being substantially equal to said givencharacteristic admittance divided by the number of said branching lines,and at least one separate frequency selective means associated with eachof said branching lines.

3. The filter according to claim 2 wherein the length of each of saidbranching lines is substantially an integral multiple of one-halfwavelength at said midfrequency.

4. A balanced strip transmission line filter capable of passingmicrowave energy extending over a given band of frequencies comprising,in combination, a pair of extended conductive surfaces in spacedparallel relationship, a thin strip of conductive material having asubstantially uniform width disposed between said surfaces in parallelrelation thereto and conductively insulated therefrom, a plurality ofconductive strip stubs conductively connected to said strip at intervalssubstantially equal to an odd multiple of one-quarter wavelength at themidfrequency of said band of frequencies, said stubs having lengthssubstantially equal to an odd multiple of one-quarter wavelength at saidmidfrequency and widths substantially greater than that of said strip,said stubs being connected to said strip at a region along their lengthsintermediate their ends, and means for conductively shorting one end ofeach of said stubs to said conductive surfaces.

5. A balanced strip transmission line filter for electromagnetic waveenergy comprising, in combination, a pair of extended conductingsurfaces in spaced parallel relationship, first and secondlongitudinally spaced thin strips of conducting material disposedbetween said surfaces in parallel relation thereto and conductivelyinsulated therefrom, said first and second strips being dimensionallyproportioned to offer a given characteristic admittance to said energy,a plurality of branching strips conductively connecting adjacent ends ofsaid first and second strips, each of said branching strips beingdimensionally proportioned to offer a characteristic admittance to saidenergy which is substantially equal to said given characteristicadmittance divided by the number of said branching strips, and separatefrequency selective means associated with each of said branching strips.

6. The filter according to claim 5 wherein two branching strips areutilized and wherein said frequency selective means comprises at leastone tapped stub element.

7. A filter capable of passing microwave energy over a given band offrequencies comprising, in combination, a uniform transmission linehaving a given characteristic admittance, a plurality of tappedtransmission line stubs shunted across said line at intervalssubstantially equal to an odd multiple of one-quarter wavelength at themidfrequency of said given band of frequencies, each of said stubshaving a length substantially equal to an odd multiple of one-quarterWavelength at said midfrequency, each of said stubs having oneopen-circuited and one short-circuited end, the connection between saiduniform line and each of said stubs being intermediate said ends, andwherein the characteristic admittance of each of 10 said stubs is atleast twice the characteristic admittance of said uniform line.

References Cited UNITED STATES PATENTS 2,819,452 1/1958 Arditi 333-842,859,417 11/1958 Arditi 333-84 2,915,716 12/1959 Hattersley 333 732,945,195 7/1960 Matthaei 333-73 2,964,718 12/1960 Packard 333-73 102,984,802 5/1961 Dyer 333-43 FOREIGN PATENTS 798,629 7/1958 GreatBritain.

15 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

Disclaimer 3,345,589.Gemld 0. Di Piazza Lake Hiawatha, N.J. TRANSMISSIONLINE TYPE MICROWAVE FILTER. Patent dated Oct. 3, 1967.

Disclaimer filed June 5, 1972, by the assignee, Bell TelephoneLaboratories Incorporated.

Hereby enters this disclaimer to claims 1, 4 and 7 of said patent.

[Oyficz'al Gazette January 2, 1.973.]

1. A FILTER CAPABLE OF PASSING MICROWAVE ENERGY OVER A GIVEN BAND OFFREQUENCIES COMPRISING, IN COMBINATION, A UNIFORM TRANSMISSION LINEHAVING A GIVEN CHARACTERISTIC ADMITTANCE, A PLURALITY OF TAPPEDTRANSMISSION LINE STUBS SHUNTED ACROSS SAID LINE AT INTERVALSSUBSTANTIALLY EQUAL TO AN ODD MULTIPLE OF ONE-QUARTER WAVELENGTH AT THEMIDFREQUENCY OF SAID GIVEN BAND OF FREQUENCIES, EACH OF SAID STUNSHAVING A LENGTH SUBSTANTIALLY EQUAL TO AN ODD MULTIPLE OF ONE-QUARTERWAVELENGTH AT SAID MIDFREQUENCY, EACH OF SAID STUBS HAVING ONEOPEN-CIRCUITED AND ONE SHORT-CIRCUITED END, THE CONNECTION BETWEEN SAIDUNIFORM LINE AND EACH OF SAID STUBS BEING INTERMEDIATE SAID ENDS, ANDWHEREIN THE CHARACTERISTIC ADMITTANCE OF EACH OF SAID STUBS ISSUBSTANTIALLY GREATER THAN THAT OF SAID UNIFORM LINE.